Thrombocytopenia is defined as a platelet count below 150.times.10.sup.9 per liter, and can be due to a number of different disorders, including, for example, impaired production of platelets by the bone marrow, platelet sequestration due to splenomegaly, or increased destruction of platelets in the peripheral circulation. Based upon their underlying cause, the throbocytopenias can be divided into different categories [Schaefer, A. I., "Thrombocytopenia and Disorders of Platelet Function" pp. 1041-1049 in Internal Medicine, 3rd Edition, John J. Hutton et al., Eds., Little Brown and Co., Boston/Toronto/London, 1990]. One of these categories, immune (autoimmune) thrombocytopenic purpura (ITP) is an uncommon illness caused by the production of antibodies to platelets and the subsequent destruction of platelets by the reticuloendothelial system. The autoantibody is usually IgG although other immunoglobulins have also been reported. The clinical bleeding manifestations of ITP depend on the severity of the condition, and possible associated coagulation effects. In severe cases, bleeding can be life threatening [Morris et al., Ann. Int. Med. 96, 714-717 (1982)]. Chronic ITP is usually accompanied by increased megakaryocytes in the bone marrow and frequently by an elevated platelet-associated immunoglobulin G (PAIgG) [Bussel et al., Blood 62-2, 480-486 (1983)].
ITP is also a common complication of HIV infection [Morris et al., Ann. Intern. Med. 96,714-717 (1982)], which can occur at any stage of its natural history, both in patients diagnosed with the Acquired Immune Deficiency Syndrome (AIDS), those with AIDS-related complex, and those with HIV infection but without AIDS symptoms.
AIDS is a transmissible disease characterized by a profound deficiency of cellular immune function and the occurrence of opportunistic infection and malignancy. In 1983, a retrovirus was identified in association with this syndrome [Barre-Sinoussi, F., Chermann, J. C., Rey, F. et al., Science 220, 868-871 (1983); and Gallo, R. C., Salahuddin, S. Z., Popovic, M., et al., Science 224, 500-503 (1984)]. Subsequently, there have been numerous isolations of related retroviruses from patients in different geographical areas, and the etiologic role of this retrovirus has been clearly established. Whereas initially these isolates were variously referred to as lymphadenopathy associated virus (LAV), human T cell lymphotropic virus-III (HTLV-III) or AIDS associated retrovirus (ARV), today the accepted terminology is human immunodeficiency virus (HIV), which has more subtypes, e.g. HIV type-1 (HIV-I) and HIV type-2 (HIV-2).
The primary immunologic abnormality resulting from infection by HIV is the progressive depletion and functional impairment of T lymphocytes expressing the CD4 cell surface glycoprotein (H. Lane et al., Ann. Rev. Immunol. 3:477 [1985]). CD4 is a nonpolymorphic glycoprotein with homology to the immunoglobulin gene superfamily (P. Maddon et al., Cell 42:93 [1985]). Together with the CD8 surface antigen, CD4 defines two distinct subsets of mature peripheral T cells (E. Reinherz et al., Cell 19:821 [1980]), which are distinguished by their ability to interact with nominal antigen targets in the context of class I and class II major histocompatibility complex (MHC) antigens, respectively (S. Swain, Proc. Natl. Acad. Sci. 78:7101 [1981]; E. Engleman et al., J. Immunol. 127:2124 [1981]; H. Spitz et al., J. Immunol. 129:1563 [1982]; W. Biddison et al., J. Exp. Med. 156:1065 [1982]; and D. Wilde et al., J. Immunol. 131:2178 [1983]). For the most part, CD4 T cells display the helper/inducer T cell phenotype (E. Reinherz, supra), although CD4 T cells characterized as cytotoxic/suppressor T cells have also been identified (Y. Thomas et al., J. Exp. Med. 154:459 [1981]; S. Meuer et al., Proc. Natl. Acad. Sci. USA 79:4395 [1982]; and A. Krensky et al., Proc. Natl. Acad. Sci. USA 79:2365 [1982]). The loss of CD4 helper/inducer T cell function probably underlies the profound defects in cellular and humoral immunity leading to the opportunistic infections and malignancies characteristic of AIDS (H. Lane supra).
The clinical spectrum of HIV infection is very complex, and the current therapy is directed toward the underlying retroviral infection as well as towards the specific opportunistic infections and malignancies that are associated with the syndrome.
Although the mechanism of HIV-associated ITP is unknown, it is believed to be different from the mechanism of ITP not associated with HIV infection in more characteristics.
In a study by Walsh et al. [N. Engl. J. Med. 311, 635-639 (1984)]ITP in 33 homosexual men who were presumed to be HIV-positive, was compared with non-HIV-related chronic ITP in 15 women and eight men. Individuals with HIV-associated ITP were reported to have 3.8-fold higher levels of platelet associated IgG and 4.2-fold higher levels of platelet-associated complement than did the patients with non-HIV-related ITP. Platelet eluates from 12 of 15 patients with non-HIV-related ITP bound platelets at a mean dilution titer of 1:8, whereas those from only one of the 10 individuals with HIV-associated ITP bound platelets at a titer of 1:2. Also, IgG in the serum of three patients with HIV-associated ITP was not capable of binding platelets, whereas IgG from two patients with non-HIV-associated ITP could bind platelets. Circulating immune complexes (CICs) were detected by the polyethylene glycol precipitation method in 88% of the individuals having HIV-associated ITP, and in 79% the CICs were capable of binding to platelets. CICs were not found in any of five patients with non-HIV-associated ITP. These and similar findings have led to the hypothesis that HIV-related ITP results from the deposition of immune complexes and complement on platelets. The immune complex deposition occurs on platelet Fc receptors. Monocytes or macrophages would then bind to free Fc domains of exposed IgG molecules or to platelet-bound complement by the C3b receptors.
An alternative hypothesis is that HIV-associated ITP results from an autoimmune process associated with the presence of a serum antibody that binds to a target platelet antigen of 25,000 kilodaltons (kD). There is a suggestion that the 25 kD protein resembles part of a sequence from the HIV polypeptide. Molecular mimicry between HIV and platelet antigens may play a role in the induction of ITP in HIV-infected individuals. This hypothesis is essentially based on a study by Stricker et al., N. Engl. J. Med. 313, 1375-1380 (1985). The authors studied ITP in patients including 30 homosexual men, 18 non-homosexual individuals with ITP, 12 patients with nonimmune thrombocytopenia and 16 homosexual men with normal platelet counts diagnosed to have either AIDS-related complex (ARC) or AIDS. Using a sensitive immunoblot technique, an antibody was identified in the sera of 9 of 30 patients with HIV-associated ITP that bound to a 25 kilodalton antigen on platelets. This activity was not found in the sera of patients with non-HIV-associated ITP or nonimmune thrombocytopenia. The serum binding activity for the 25 kilodalton platelet antigen was found in the IgG fraction and was mediated by the F(ab).sub.2 fragment.
For further details, I refer to a recent review article by Ratner, L., [Am. J. Med. 86, 194-198 (1989)], and the references cited therein.
The therapeutic approach to the treatment of patients with ITP is dictated by the severity and urgency of the clinical situation. The treatment is similar for HIV-associated and non-HIV-related ITP, and although a number of different therapeutic approaches have been used in the clinical practice, the therapy remains controversial.
In many adult patients with (HIV-associated) ITP thrombocytopenia is mild (the platelet count is more than about 20-30.times.10.sup.9 /liter) and no bleeding occurs. In such cases monitoring with no therapy is considered to be the best option.
Platelet counts in patients diagnosed with ITP have been successfully increased by glucocorticoid (e.g. prednisolone) therapy, however in most patients the response is incomplete, or relapse occurs when the glucocorticoid dose is reduced or its administration is discontinued. Based upon studies with patients having HIV-associated ITP, some investigators have suggested that glucocorticoids may predispose to AIDS. This approach is usually followed if the platelet count is below 20.times.10.sup.9 /liter or if bleeding occurs.
Good results were reported with splenectomy. Splenectomy removes the major site of platelet destruction and a major source of autoantibody production in many patients. This procedure results in prolonged treatment-free remissions in a large number of patients. However, since surgical procedures are generally to be avoided in immune compromised patients, splenectomy is recommended in severe cases of HIV-associated ITP only, in patients who fail to respond to 2 to 3 weeks of glucocorticoid treatment, or do not achieve sustained response after discontinuation of glycocorticoid administration. Based upon current scientific knowledge, it is unclear whether splenectomy predisposes patients to AIDS.
Significant responses have been documented to high doses (about 500 mg/kg body weight and above) of immunoglobulin [Rarick et al., Proceedings of the Fourth International Conference on AIDS, Stockholm, (1988) p. 7642; Sanderson et al., ibid, p. 7646; Panzer et al., Transfusion 26, 69-72 (1986); Delfraissy et al., Ann. Intern. Med. 103, 478-479 (1985); Ordi et al., Ann. Intern Med. 104, 282-283 (1986)], and to anti-Rh immunoglobulin treatment [Oksendhendler et al., Blood 71, 1499-1502 (1988); Bierling et al., Ann. Intern. Med. 106, 773-774 (1987); Durand et al., Lancet 2:40 (1986); Biniek et al., Lancet 2:627 (1986)].
Certain cytotoxic agents, e.g. vincristine, and azidothimidine (AZT, zidovudine) have also shown some promising effects, however, the results are very preliminary.
AZT, a reverse transcriptase inhibitor nucleoside analogue, has been extensively tested in AIDS therapy and was found to result in a decreased incidence of opportunistic infections and increased survival in AIDS patients [Fischl et al., N. Engl. J. Med. 317, 185-191 (1987)]. However, it is clear that AZT therapy is associated with significant dose-limiting toxicities, including anemia and other hematologic toxicity, particularly in individuals with advanced HIV infection [Richman et al., N. Engl. J. Med. 317, 192-197 (1987)]. Similarly, the toxicity of cytotoxic agents, such as vincristine is of great concern.
CD4-Immunoglobulin hybrid molecules are known in the art, and are, for example, disclosed in co-pending U.S. application Ser. No. 07/250,785 (filed 28 Sep. 1988), which is a continuation-in-part of Serial No. 07/104,329 (filed 2 Oct. 1987), and in the counterpart PCT Application Publication No. WO 89/02922 (published 6 Apr. 1989). Fusion proteins of immunoglobulins of the IgM, IgG1 or IgG3 class, wherein the variable region of the light or heavy chain has been replaced with CD4 or CD4 fragments are also described in EP 325,262. Such fusion proteins have so far been recommended for the treatment of HIV infection or for use in assays for detection of the HIV virus. According to WO 89/02922 the typical dose for the treatment of HIV infection corresponds to about 100 .mu.g/kg/day soluble CD4. According to EP 325,262 the dose of the CD4-immunglobulin molecule may vary from 0.01 mg/kg to 50 mg/kg, 0.1 mg/kg to 1.0 mg/kg being preferred.
The results of a phase 1 clinical study initiated to evaluate the safety and pharmacokinetics of a recombinant soluble CD4 (rCD4) molecule in the treatment of AIDS were reported by Kahn et al. in Annals of Internal Medicine 112(4), 254-261 (1990). The patients were enrolled at dose levels of 1, 10, 30, 100, and 300 .mu.g/kg body weight of rCD4 administered intravenously. One of the patients had a platelet count of 68.times.10.sup.9 /liter at entry, and during the rCD4 treatment had a progressive decrease to 41.times.10.sup.9 /liter at the time that disseminated histoplasmosis with bone marrow involvement was diagnosed.
It is an object of the present invention to provide an efficient treatment for HIV-associated ITP which is devoid of the disadvantages of the hitherto known approaches.